Gas Turbine Engine Start-up

Starting a Gas Turbine Engine

• Gas turbine engines require two separate systems for a successful start
• Provision must be made to rotate the compressor and turbine
• Rotation ensures adequate airflow into the combustion system for proper fuel mixing via spray nozzles
• Sufficient engine speed is required to reach engine idling RPM

Ignition Systems

• Ignition of the air/fuel mixture in the combustion system must be provided
• The two starting systems operate simultaneously during engine start
• The engine can be motored without ignition for maintenance
• The ignition system can operate alone for relighting mid-flight

Methods of Starting Jet Engines

• The starting procedure is the same across all jet engines
• Methods to achieve the procedure can vary
• A starter motor must generate high torque
• The starter motor needs to transfer the torque to the rotating engine assembly
• Smooth acceleration from rest should be provided
• Acceleration should occur until the gas flow offers sufficient power for the engine turbine to take over

Types of Starters

• Electric Starter
• Cartridge Starter
• Isopropyl-Nitrate Starter
• Air Starter
• Gas Turbine Starter
• Hydraulic Starter

Electric Starters

• Electric starters typically use a direct current (D.C.) electric motor
• These motors are linked to the engine using a reduction gear and ratchet mechanism or clutch
• The clutch automatically disengages when the engine achieves self-sustaining speed
• It provides power to operate the ignition system
• When the starter load decreases after engine start, or when the time cycle ends, electrical supply is automatically cut

Cartridge Starters

• Cartridge starting can be used in military engines
• It is a quick, independent starting method
• A starter motor features an impulse-type turbine
• The turbine is propelled by high-velocity gases produced by a burning cartridge
• An electrically fired detonator starts the cartridge burning
• The turbine's power goes through a reduction gear
• An automatic disconnect mechanism rotates the engine

Isopropyl-nitrate Starters

• Provides high power output
• It has rapid starting characteristics
• This starter type uses a turbine
• The turbine transmits power to the engine through a reduction gear
• High-pressure gases from isopropyl-nitrate combustion rotates the turbine
• Fuel is sprayed into a combustion chamber then electrically ignited with a high energy ignition system
• A pump delivers fuel from a storage tank to the combustion chamber
• An air pump removes fumes from combustion chamber before each fresh start

Air Starters

• Air starting sees use on most commercial and some military jet engines
• Offers advantages like being light, simple, and economical
• An air starter motor transfers power via reduction gear and clutch
• It transfers power to the output shaft that connects to the engine
• The starter turbine is rotated by air
• The air is supplied by an external ground source, an auxiliary power unit (APU), or cross-feed from a running engine
• An electrically operated control and pressure reducing valve controls the air supply
• The clutch automatically disengages when the engine rpm reaches idling speed, stopping starter rotation

Gas Turbine Engine Starters

• Used for some jet engines
• Completely self-contained
• Possesses its own fuel, ignition and starting system (typically electric or hydraulic)
• A self-contained oil system is included
• Cost-effective to operate
• Produces high power relative to low weight
• Features a small, compact gas turbine engine
• Usually includes a turbine-driven centrifugal compressor
• A reverse flow combustion system operates within
• Has a mechanically independent free-power turbine
• The free-power turbine connects to the main engine
• This occurs via a two-stage epicyclic reduction gear, automatic clutch, and output shaft

Hydraulic Starters

• Used for starting smaller jet engines
• Torque is transmitted to the engine in various ways
• A typical system includes a reduction gear and clutch setup
• Hydraulic pressure derived from a ground supply unit rotates the pump/starter

Engine Ignition Systems

• A turbine engine ignition system consists of:
• A high voltage exciter
• A high voltage transmission lead
• An igniter plug
• The exciter gets power from the aircraft electrical system (DC or AC)
• A permanent magnet alternator (PMA) can be used instead
• The exciter produces a high voltage pulse
• The pulse travels through the igniter lead to the plug
• The igniter plug arcs, causing a spark that ignites the fuel/air mixture in the combustion chamber

Modern Ignition Systems

• Modern gas turbine engine ignition systems are capacitor discharge type
• Either of the Intermittent Duty or Extended Duty cycles can be used
• Intermittent Duty types employ sufficient high current
• This can cause units to overheat and damage
• The Intermittent Duty types have a restricted duty cycle based on operating time followed by a cooling period
• Extended Duty types feature long duty cycles
• Some Extended Duty types are rated for continuous duty with no time limits
• Ignition isn't needed after a normal start
• The ignition system is then deactivated
• High-energy (H.E.) ignition is used to start all jet engines
• A dual system is always installed
• Each system has an ignition unit
• An igniter plug is connected to its ignition unit
• The set of two plugs are placed in the combustion system at different positions
• Ignition units are measured in Joules
• Units give varied outputs to meet requirements
• A high output value (12 joules for example) ensures satisfactory relight at high altitudes or start

Alternative Ignition Units

• A low value output between 3 to 6 joules is preferred in conditions like icing or heavy precipitation
• It results in a longer life for the igniter plug and ignition unit

Igniter Plugs

• Two primary igniter plug types feature;
• Constricted or constrained air gap type
• Shunted surface discharge type
• The air gap type is like a reciprocating engine spark plug, but has a larger air gap

Voltage in Igniters

• A potential difference of approximately 25,000 volts is needed to ionize the gap before a spark
• High voltage requires excellent circuit insulation
• The surface discharge igniter plug features a semiconducting pellet
• This allows electrical leakage from the central high tension electrode to the grounded body
• The ionizing surface of the pellet makes a low resistance path for energy stored in the capacitor
• A high-intensity flashover happens between the electrode and body and requires approximately 2000 volts to work

Internal Air System

• Refers to airflows that do not directly contribute to engine thrust
• It provides air for the aircraft's systems
• Can use up to a fifth of total engine core mass airflow for various tasks
• To limit engine perfomance losses, air is tapped as early as possible from the compressor
• Tapping position varies according to the demands of each function

Cooling Considerations in Jet Engines

• It has to be ensured that various parts of the engine and accessories do not absorb detrimental heat
• The combustor and turbine require air cooling
• Cooling air controls temperature of compressor shafts and discs with cooling or heating
• Equal temperature distribution ensures improves thermal efficiency
• Ensures minimal blade tip and seal clearances

Turbine Cooling

• High thermal efficiency is dependent upon high entry temperature
• High entry temp is limited by the turbine blade and nozzle materials
• Components must be continuously cooled
• Continuous cooling means their temperatures can exceed the material's melting point without risk to blade and vane integrity
• Heat transfer needs the cooling of the discs
• Disc cooling prevents thermal fatigue and uncontrolled expansion and contraction
• The most popular methods of cooling are using engine bleed air
• Convection cooling/film cooling
• Transpiration cooling
• Compressor bleed air is directed inside through the hollow blades
• It then exits through holes in the tip, leading edge, and trailing edge in convection cooling
• Transpiration cooling is restricted to only stationary nozzle vanes
• They are connected to bleed air
• The air exits through the porous material
• Early engines employed single pass internal (convection) cooling
• Impingement cooling of vanes with external air film cooling of vanes and blades arrived later

Accessory Cooling

• Accessories generate substantial heat needing cooling
• Accessories like electrical generators require their own cooling circuit
• When air acts a coolant, it can be derived from the compressor
• Atmospheric air that is ducted from intake louvres in the engine cowlings can also supply the coolant
• When accessory unit is cooled with atmospheric air in flight, another circuit is generally required
• It's needed for use during static ground running when there's no atmospheric coolant
• Accomplished by feeding compressed air via nozzles to the air outlet duct of the accessory
• Air velocity forms a low pressure area and forms an ejector
• It induces atmospheric air flow through intake louvres
• Compressor air operates the ejector system
• A valve controls the flow of air

Gas Turbine Engine Sealing Methods

• Choice relies on:
• Surrounding temperature
• Pressure
• Wearability
• Heat generation
• Weight
• Space
• Ease of manufacture
• Ease of installation and removal
• Seals are used:
• To stop oil leakage from engine bearings
• To control cooling airflows
• To prevent entry of mainstream gas into turbine disc cavities

Types of Seals

• Labyrinth
• Ring
• Hydraulic
• Carbon
• Brush